In accordance with the recent miniaturization and thickness reduction of electronic devices, there is strong demand for miniaturization and thickness reduction of electronic parts or devices that are used in electronic devices. On the other hand, due to an increase in speed and high integration in LSIs, such as CPUs, there are cases in which an electric current of several A to several tens of A is supplied to a power supply circuit supplied in an LSI. Therefore, there is demand for suppression of inductance degradation caused by direct current (DC) superposition as well as miniaturization and thickness reduction even in coil parts. Furthermore, there is additional demand for a low loss in a high frequency range as the operating frequency is increased. In addition, it is also desired that simple-shaped elements can be assembled by a simplified process from the viewpoint of cost reduction. That is, there is demand for supply of coil parts that can counteract a large electric current in a high frequency range and be miniaturized and reduced in thickness with lower costs.
The DC superposition characteristics are improved as the saturation magnetic flux density is increased in cores used in such coil parts. In addition, an increase in the magnetic permeability allows a high inductance value to be obtained, but degrades the DC superposition characteristics since a dust core becomes liable to be magnetically saturated. Therefore, a desirable range of the magnetic permeability is selected according to use. In addition, the magnetic loss of a core is desirably low.
An ordinary coil part in practical use is an element having a so-called EE-type or EI-type ferrite core and a coil, but the magnetic permeability of the ferrite material is high and the saturation magnetic flux density is low in this element. Therefore the inductance value is significantly degraded by magnetic saturation, and the DC superposition characteristics are deteriorated. It is possible to provide voids in the magnetic path direction of the core and use the element with lowered apparent magnetic permeability in order to improve the DC superposition characteristics, but oscillation of the core occurs in the void portions when the element is driven under an alternative current, thereby generating noise sound. In addition, since the saturation magnetic flux density of the ferrite material is still low even when the magnetic permeability is lowered, it is difficult to achieve fundamental improvement.
Therefore, Fe-based metallic magnetic materials such as a Fe—Si-based, Fe—Si—Al-based, Fe—Ni-based alloy having a higher saturation magnetic flux density than ferrite are used as a core material. However, since these metallic magnetic materials have a low electrical resistivity, when the operating frequency range is increased to several hundred kHz to several MHz as recently, the eddy-current loss is increased, and the materials cannot be used in a bulk state. Therefore, a dust core having metallic magnetic powder insulated by powdering a metallic magnetic material and a resin interposed between the metallic magnetic powder particles has been developed. Generally, such a dust core is manufactured by pressing a granular compound composed of metallic magnetic powder and a resin. A coil can be buried in a dust core by integrally molding the compound and the coil, whereby a coil-buried magnetic element can be manufactured. Since a coil-buried magnetic element is manufactured by integrally molding a coil and a compound, the manufacturing process is simple, and cost reduction can be achieved.
In addition, in comparison to an assembled magnetic element manufactured by assembling a coil and a dust core, dead spaces, such as a dimensional allowance created between the coil and the dust core in the assembled magnetic element, can be packed with the dust core in the coil-buried magnetic element, and therefore the coil-buried magnetic element can shorten the magnetic path length and extend the magnetic path cross section, and is superior in terms of the miniaturization and thickness reduction of the element.
On the other hand, since the coil and the dust core are in contact with each other in the coil-buried magnetic element, if insulation breakdown occurs in the dust core when a voltage is applied between the coil terminals, a short circuit is induced between the coil and the coil in the dust core. In addition, when a coil-buried magnetic element in which a dust core having a low electrical resistivity is used in a power supply circuit or the like, there is concern that degradation of circuit efficiency may be induced by leakage current. Therefore, there is demand for the dust core to have electrical resistivity and voltage resistance suitable for use of the coil-buried magnetic element.
Meanwhile, for example, PTL 1 and PTL 2 are known as related art documents concerning the invention of the present application. PTL 1 discloses a dust core that is composed of metallic magnetic powder, an electrically insulating material and a thermosetting resin, and has favorable magnetic properties and voltage resistance, and a method of manufacturing a coil-buried magnetic element using the same. However, the dust core in PTL 1 has an electrical resistivity (DC 50 V) that is abruptly lowered after a high-temperature heat resistance test and has a problem of reliability. The reason for the problem can include the fact that the resin gradually contracts overreactions after a thermosetting treatment due to aging variation during a high-temperature heat resistance test, and the distance between the metallic magnetic powder particles is shortened or the metallic magnetic powder particles comes into contact with each other in the dust core in PTL 1. PTL 2 discloses a dust core in which the electrical resistivity (DC 50 V) is prevented from being lowered after a high-temperature heat resistance test by using an organic binding material having a molecular weight of 200 to 8000 for an insulating film on the surface of the metallic magnetic powder particles.
However, there is demand for coils that are used in some vehicle ECU-driving circuits to have a voltage resistance of about 100 V after a high-temperature heat resistance test. Since the coil-buried magnetic elements using the dust cores in the related art do not have a voltage resistance of 100 V after the high-temperature heat resistance test, an object is to further increase the voltage resistance of dust cores.